The goal of this research program is to use noninvasive techniques to gain a better understanding of cochlear nonlinearity in humans with normal hearing and with hearing loss. This objective will be approached through a combination of acoustical (middle-ear transfer functions, distortion product otoacoustic emissions, DPOAE), electrophysiological (auditory brainstem response, ABR), and behavioral measurements. The variability in DPOAE measurements will be studied by evaluating the influences of behavioral threshold, middle-ear forward and reverse energy transmission, contributions from the reflection source, and contributions resulting from basal spread of the inter-modulation source on the variability in response levels in humans. Studies of the nonlinearity at the cochlear apex and base will focus on comparisons of compression, response growth, tuning and cochlear amplification for low- and high-frequency cochlear regions. The extent to which behavioral simultaneous masking can be attributed to mechanical suppression in the cochlea will be studied by comparing simultaneous masking and DPOAE suppression in the same subjects. Behavioral and mechanical measurements will be compared in terms of masking/suppression growth, tuning, and cochlear-amplifier gain. The relation between mechanical and neural (ABR) estimates of response growth, tuning and cochlear-amplifier gain will be explored, including measurements in both normal and impaired ears. Here, ABR measurements will make in a simultaneous masking paradigm in much the same way as DPOAE suppression measurements are made, extending the range of stimulus levels over which cochlear processes can be examined. A physically based model of cochlear mechanics will be used to simulate mechanical, neural, and behavioral measurements related to cochlear function, with particular attention to nonlinear processes. The plan of study should increase our knowledge of human cochlear processes and has the potential to lead to the development of better diagnostic strategies for patients with heating loss, especially infants, young children and patients with developmental disabilities who may be unable to provide voluntary responses to sound. The research program combines studies that increase our understanding of underlying process with more clinically oriented efforts that should help to increase the translational nature of the work.